(1) Field of the Invention
This invention relates to the casting of metal strip articles by means of continuous strip casting apparatus of the kind that employ continuously moving elongated casting surfaces and side dams that confine the molten and semi-solid metal to the casting cavity formed between the moving casting surfaces. More particularly, the invention relates to the side dams themselves, and particularly, but not exclusively, to those intended for the casting of aluminum and alloys thereof.
(2) Description of the Related Art
Metal strip articles (such as metal strip, slab and plate), particularly those made of aluminum and aluminum alloys, are commonly produced in continuous strip casting apparatus. In such apparatus, molten metal is introduced between two closely spaced (usually actively cooled) elongated moving casting surfaces forming a casting cavity, and is confined within the casting cavity until the metal solidifies (at least sufficiently to form an outer solid shell). The solidified strip article, which may be produced in indefinite length, is continuously ejected from the casting cavity by the moving casting surfaces. One form of such apparatus is a twin-belt caster in which two confronting belts are rotated continuously and molten metal is introduced by a launder or injector into a thin casting cavity or mold formed between the confronting regions of the belts. An alternative is a rotating block caster in which the casting surfaces are formed by blocks that move around fixed paths and align with each other within the casting cavity. In both kinds of apparatus, the molten metal is introduced at one end of the apparatus, conveyed by the moving belts or blocks for a distance effective to solidify the metal, and then the solidified strip emerges from between the belts or blocks at the opposite end of the apparatus.
In order to confine the molten and semi-solid metal within the casting cavity, i.e. to prevent the metal escaping laterally from between the casting surfaces, it is usual to provide metal dams at each side of the apparatus. For twin-belt and rotating block casters, side dams of this kind can be formed by a series of metal blocks joined together to form a continuous line or chain extending in the casting direction at each side of the casting cavity. These blocks, normally referred to as side dam blocks, are trapped between and move along with the casting surfaces and are recirculated so that blocks emerging from the casting cavity exit move around a guided circuit and are fed back into the entrance of the casting cavity. The blocks are guided around this circuit by means of a metal track, or similar guide, on which the blocks can slide in a loose fashion that allows for limited movement between the blocks, especially as they move around curved parts of the circuit outside the casting cavity.
A problem with side dams made of blocks of this kind is that it is sometimes desired to change the through-thickness convergence of the belts, i.e. to make the casting cavity thinner at its exit than at its entrance (referred to as convergent) in order to extract more heat from the metal slab, or alternatively, to make the casting cavity thicker at the exit (referred to as divergent) in order to extract less heat from the metal slab. A requirement that the belts also drive the side dam blocks through the casting cavity may limit the extent to which the casting belts can be changed in this way.
The casting belts or blocks extract heat from the molten metal passing through the casting cavity, but heat is also extracted at the sides of the cavity where the molten metal contacts the side dam blocks which are usually made of a heat conductive material such as cast iron or mild steel. This heat extraction at the sides of the cavity often changes the microstructure and thickness of the slab in those areas, resulting in undesirable side-to-center non-uniformity of the cast metal slab.
U.S. Pat. No. 4,869,310 issued to Yanagi et al. on Sep. 26, 1989 discloses a twin-belt casting apparatus having side dams provided by moving side dam blocks as explained above. For comparison with the moving side dam blocks, however, this patent also shows the use of fixed side dams in FIGS. 7 and 8 of the patent. These fixed side dams extend for the full length of the casting cavity and are said to be liable to cause seizure when the metal solidifies. Also, it is said that a change in the width of the cast piece is not possible when such fixed side dams are employed.
There is therefore a need to address the problems mentioned above.